There is known a method of forming a random vibration spectrum (cf. USSR Inventor's Certificate No. 862,018, Cl. G 01 M 7/00) wherein narrow-band filters are utilized to successively produce vibrations of an article by the use of uncorrelated narrow-band random signals in each band of a predetermined frequency range. The frequency bands of one group are designated with even numbers, the frequency bands of the other group being designated with odd numbers. A random vibration spectrum is assigned to suit a particular article under test as certain values of spectral vibration acceleration density in the specified frequency bands. The "vibrator-article" system is created after the article to be tested is secured on a vibrating table. At the initial stage said system is fed with random signals of the first group, and the level of random signals of said group is adjusted to conform to the present values of spectral vibration acceleration density, the subsequent step being computation of correction factors C.sub.n from the expression ##EQU1## where q.sub.n is the preset value of spectral vibration acceleration density in the nth frequency band relating to the first group of the frequency bands;
.phi..sub.n (.omega.) is the amplitude-frequency response of the nth narrow-band filter in the nth frequency band; PA1 .psi..sub.l (.omega..sub.n) is the amplitude-frequency response of the "vibrator-article" system at the midband frequency in the nth frequency band; and PA1 .DELTA..OMEGA. is the predetermined frequency range. PA1 .psi..sub.l (.omega.) is the amplitude-frequency response of the "vibrator-article" system at the 1th check point; and PA1 .DELTA..omega..sub.i is the ith band of the predetermined frequency range;
Next the levels of the transmitted random signals are corrected by multiplying their values obtained after the adjustment at the first stage by the computed correction factors C.sub.n. The corrected random signals are stored in the memory unit. The above operations are then performed on random signals of the second group of frequency bands. Thereafter the random signals in all bands of the predetermined frequency range are simultaneously read off from the memory unit and applied to the "vibrator-article" system.
There is also known a device for accomplishing the aforesaid method (cf. the cited USSR Inventor's Certificate) which comprises a white noise generator having its output connected to the input of a tuneable narrow-band filter which uses the white noise generator signal to successively furnish narrow-band random signals to a level adjuster whose input is connected to the output of the tuneable narrow-band filter. The output of the level adjuster is connected via the first contact of a switch to the input of the vibrator and simultaneously to the input of the memory unit. The output of the memory unit is connected to the vibrator input via the second contact of said switch.
The adjusted random signals of the first group and then of the second group are successively stored in the memory unit. Thereafter the signals in all bands of the predetermined frequency range are simultaneously read off and applied to the vibration-testing machine by the use of the switch.
The known method and device permit obtaining preset values of spectral vibration acceleration density in each band of a predetermined frequency range with due account taken of the amplitude-frequency response of the "vibrator-article" system. However, this may be done only at one point on a given article using one vibrator (and not at all check points having characteristic values of spectral vibration acceleration density in each band of a predetermined frequency range), which is generally a limiting factor,
Also known in the art is a method of forming a random vibration spectrum (cf. Y. V. Beselov, V. V. Sumarokov and V. F. Cherepov "Equipment for Reproducing and Recording Random Signals and Shocks," Leningrad, izdatelstvo Leningradskogo doma naucho-technichesky propagandi, 1979, pp 15-18, in Russian) comprising the step of choosing check points on an article under test, said points having characteristic values of spectral vibration acceleration density in each band of a predetermined frequency range. Next, the article under test is subjected to vibration in the predetermined frequency range by the use of a multichannel random signal shaper wherein each channel furnishes a narrow-band random signal, the output signal level being appropriately adjusted. The operations performed at the check points include measurements of spectral vibration acceleration density in the specified bands, computation of mean values of spectral vibration acceleration density at all the check points separately in each band of the predetermined frequency range and subsequently comparison of the obtained mean values with preset values of spectral vibration acceleration density for a particular article, the next step being utilization of error signals to adjust levels of narrow-band random signals in each channel of the multichannel shaper, the random signal spectrum at the output of said shaper equal to the sum of the narrow-band random signals characterizing the random vibration spectrum of the article under test.
The foregoing method is accomplished by a device for forming a random vibration spectrum (cf. the cited publication) comprising a multichannel shaper having several channels, each of which includes such series-connected components as a white noise generator, a bandpass filter and an adjustable amplifier, the outputs of all the adjustable amplifiers being connected to the inputs of an adder which is also a part of the multichannel shaper. The output of the adder is connected to a power amplifier connected to the input of a vibrator mounting an article under test. The device also comprises vibration pick-ups representing means for converting mechanical oscillations into an electrical signal, said means being secured at check points on the article under test. The outputs of the vibration pick-ups are connected through a switch to the input of a multichannel random signal spectrum analyzer, each channel of which corresponding to a similar channel of the multichannel shaper includes a bandpass filter, a random signal level meter, and a comparison unit. The outputs of the analyzer are connected to the control inputs of the adjustable amplifiers in similar channels of the multichannel shaper.
Wide-band random signals derived from the outputs of the white noise generators and having a uniform spectral density level in a predetermined frequency range come to narrow-band filters which separate from the wide-band random signal spectrum only those spectral components which are within their transmission band, the chosen parameters of the formed spectrum being a governing factor. The output signals of the narrow-band filters are amplified in the adjustable amplifiers and added up. The output signal of the multichannel shaper is applied to the vibrator input through the power amplifier. Random signals characterizing vibration at check points on the article under test are successively fed to the inputs of the bandpass filters of the spectrum analyzer by means of the switch. The output signals of the random signal level meter in each channel of the analyzer characterizing the spectral vibration acceleration density level are averaged with respect to the chosen check points. Thus, the signal level at the output of each analyzing channel characterizes the mean spectral vibration acceleration density level at several check points in the frequency band of a given channel. Next, the output signal of the meter in each channel of the analyzer characterizing the mean spectral vibration acceleration density in the frequency band of the given channel comes to the first input of the comparison unit, the input thereof being fed with a fixed voltage whose level determines the desired spectral vibration acceleration density level in the specified frequency band. An error signal from the output of the comparison unit is used to control the gain of the adjustable amplifier in a similar channel of the multichannel shaper.
With the known method, spectral vibration acceleration density is measured at check points by causing the article under test to vibrate in response to a random signal, a disadvantage substantially complicating the analysis, particularly when a predetermined frequency range is wide. The analysis of a random signal is characterized by errors: ##EQU2## (cf. J. S. Bendat, A. G. Piersol, Measurement and Analysis of Random Data, John Wiley & Sons Inc., New York, London, Sidney).
Measurements of levels of a random signal in channels of a multichannel shaper with respect to spectral vibration acceleration density values obtained by averaging the values of spectral vibration acceleration density at check points on a given article under test do not permit having random vibrations of the article under test with predetermined values of spectral vibration acceleration density at each check point on the article under test. Stated differently, articles may not be subjected to random vibrations equivalent of actual vibrations affecting the articles in operation.
Disadvantages of the known device are, firstly, the need for a multichannel random signal analyzer, which substantially complicates the device and gives rise to a considerable error component thereof due to random nature of a measured signal and, secondly, stringent requirements for squareness, stability and similarity of amplitude-frequency responses of similar channels of a multichannel shaper and a random signal analyzer to enable normal operation of the device. If said requirements are not met, there occurs a control error, particularly when stability of a tuning frequency of a filter is comparable with its transmission band, a condition characterized by that the nth channel of the analyzer measures the random signal level in the (n-1)th or (n+1)th frequency band and controls the signal in the nth frequency band.